The fire that engulfed the Cutty Sark last May, resulting in £10million damage, was caused by a piece of electrical equipment that was left on over the weekend, The Times has learnt.

The Metropolitan Police are expected to say today that a dust extractor was the cause of the blaze when it publishes the results of its investigation.

Officers are understood to have concluded that the power to the extractor was left on when workers at the site went home on Friday, May 18, which led to the equipment catching fire in the early hours of the following Monday. There were no sprinklers on the dry dock site in Greenwich, southeast London, because they had been removed while the ship was being repaired.

Detectives initially treated the fire as suspicious. However, a source close to the project to restore the 140-year-old ship told The Times: “The report says that electrical equipment failed mysteriously, early on Monday morning. The question is, why didn't a fire alarm go off?”

Questions may also be raised as to whether end-of-day checks carried out by Heery, the construction management company in charge of the site, would have revealed if any electrical appliances had been left on.

When the fire started, the Cutty Sark Trust was about a quarter of the way through a £25 million restoration of the vessel, which had been closed to visitors since November 2006.

Damage to all three decks, along with lengthy delays to the restoration, has added an additional £10 million to the bill, not all of which was covered by insurance.

The project, a painstaking exercise to strip the 19th-century clipper and reassemble her, using existing pieces, has been embroiled in controversy. Not every element can be reused, but no expense is being spared in trying: rusting iron is being electrolysed to reverse corrosion, the keel is sandwiched in steel, and crumbling timbers are being glued back together.

Almost 96 per cent of the Cutty Sark's hull survives from her glory days. Such rare authenticity is the reason why those behind the project are so determined to preserve as much of the original ship as possible.

Balfour Beatty Management, parent company of Heery, declined to comment last night.

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